Multiply the following complex numbers, marked as blue dots on the graph: $( e^{2\pi i / 3}) \cdot (2 e^{3\pi i / 2})$ (Your current answer will be plotted in orange.)
Solution: Multiplying complex numbers in polar forms can be done by multiplying the lengths and adding the angles. The first number ( $ e^{2\pi i / 3}$ ) has angle $\frac{2}{3}\pi$ and radius $1$ The second number ( $2 e^{3\pi i / 2}$ ) has angle $\frac{3}{2}\pi$ and radius $2$ The radius of the result will be $1 \cdot 2$ , which is $2$ The sum of the angles is $\frac{2}{3}\pi + \frac{3}{2}\pi = \frac{13}{6}\pi$ The angle $\frac{13}{6}\pi$ is more than $2 \pi$ . A complex number goes a full circle if its angle is increased by $2 \pi$ , so it goes back to itself. Because of that, angles of complex numbers are convenient to keep between $0$ and $2 \pi$ $\frac{13}{6}\pi - 2 \pi = \frac{1}{6}\pi$ The radius of the result is $2$ and the angle of the result is $\frac{1}{6}\pi$.